Electrophotographic printers are typically constructed with an image fixing station spaced downstream from an image transfer station. Print media, such as paper, is moved through the image transfer station and then through the image fixing station. At the image transfer station, an electophotographic image is transferred to the print media. At the image fixing station, the image is affixed to the print media. The print media is moved through the stations by a roller assembly located at the image fixing station and a guidance system positioned intermediate of the image transfer station and the image fixing station.
A motor is employed to drive the roller assembly and the guidance system. The motor is typically an eletromechanical stepper motor which rotates a precise amount in response to a change in current flow through its windings. The speed of the changes in current flow controls the rate of rotation of the stepper motor, and the order of the changes controls the direction of rotation. The control of the stepper motors is the focus of the present invention.
Prior art electrophotographic printers employ a microprocessor to control the operation of the stepper motors. The microprocessor is directly coupled, or coupled through an amplifying circuit, to the stepper motor. The microprocessor supplies a series of pulses to effectuate the change in current in the stepper motor and thereby change its speed or direction. To drive the stepper motor at a constant speed, the microprocessor must output a constant series of pulses. This demands a substantial amount of processing time and thus, the microprocessor is not able to perform other processing tasks. As the speed of the stepper motor increases, the microprocessor must output the pulses at a higher frequency, thereby consuming even more processing time. Eventually, as the stepper motor speed becomes high enough, the microprocessor becomes entirely dedicated to controlling the stepper motor. As a result, the microprocessor has no time to handle other printer processing tasks.
Direct control of a stepper motor consumes such a significant percentage of the microprocessor's resources that a second microprocessor is often required in the printer engine to handle other processing tasks. For example, the second microprocessor may be used to monitor the paper speed, the fuser temperature in the fuser roller, the laser power output, communicate with the raster image processor, or monitor sensors located throughout the printer (such as a toner low sensor, a media sensor, or a temperature sensor). Since two microprocessors are used in the printer engine, the prior art printers also have separate I/O devices and electronics dedicated solely to the communication between the two microprocessors.
Besides the disadvantage of consuming large amounts of processing time, stepper motors controlled by conventional controllers also suffer unpredictable behavior if the regular microprocessor interaction necessary for normal operation is lost for any reason.
One of the advantages of the present invention is to provide an electrophotographic printer that controls the operation of the stepper motor without constant interaction with the microprocessor, thereby freeing up processing time for other processing tasks. As a result, a single microprocessor may be employed to handle all of the printer engine tasks, including control of the stepper motor. Another advantage of the present invention is that the behavior of the stepper motor in the event of a microprocessor failure is completely predictable.
These and other advantages of the present invention will become apparent upon reading the following detailed description of a preferred embodiment.